On the host/hypervisor:

1. 

[root@tb /bhyve]# ls -lh bhyve-1-10
-rw-r--r--  1 root  wheel    32G Feb  2 11:48 bhyve-1-10
[root@tb /bhyve]# du -h bhyve-1-10
 12G	bhyve-1-10
[root@tb /bhyve]# truncate -s +32G bhyve-1-10
[root@tb /bhyve]# ls -lh bhyve-1-10
-rw-r--r--  1 root  wheel    64G Feb  2 11:48 bhyve-1-10
[root@tb /bhyve]# du -h bhyve-1-10
 12G	bhyve-1-10

On the vm/guest

2.

[root@bhyve-1-10 ~]# gpart show
=>      34  67108797  vtbd0  GPT  (32G)
        34       128      1  freebsd-boot  (64k)
       162  59573309      2  freebsd-ufs  (28G)
  59573471   7535360         - free -  (3.6G)

root@bhyve-1-10:~ # df -k
Filesystem                                               1024-blocks     Used      Avail Capacity  Mounted on
/dev/vtbd0p2                                                28840536  5546448   20986848    21%    /
devfs                                                              1        1          0   100%    /dev
tmpfs                                                        3442736        4    3442732     0%    /tmp

[root@bhyve-1-10 ~]# reboot

3. after reboot

root@bhyve-1-10:~ # gpart show
=>      34  67108797  vtbd0  GPT  (64G) [CORRUPT]
        34       128      1  freebsd-boot  (64k)
       162  59573309      2  freebsd-ufs  (28G)
  59573471   7535360         - free -  (3.6G)

root@bhyve-1-10:~ # gpart recover vtbd0
vtbd0 recovered
root@bhyve-1-10:~ # gpart show
=>       34  134217661  vtbd0  GPT  (64G)
         34        128      1  freebsd-boot  (64k)
        162   59573309      2  freebsd-ufs  (28G)
   59573471   74644224         - free -  (35G)
   
root@bhyve-1-10:~ # gpart resize -i2 vtbd0
vtbd0p2 resized
root@bhyve-1-10:~ # gpart show
=>       34  134217661  vtbd0  GPT  (64G)
         34        128      1  freebsd-boot  (64k)
        162  134217533      2  freebsd-ufs  (64G)
		
root@bhyve-1-10:~ # growfs -s 134217533 /dev/vtbd0p2
Device is mounted read-write; resizing will result in temporary write suspension for /.
It's strongly recommended to make a backup before growing the file system.
OK to grow filesystem on /dev/vtbd0p2, mounted on /, from 28GB to 64GB? [Yes/No] Yes
super-block backups (for fsck -b #) at:
 60265472, 61547712, 62829952, 64112192, 65394432, 66676672, 67958912, 69241152, 70523392, 71805632, 73087872, 74370112, 75652352, 76934592, 78216832, 79499072, 80781312, 82063552, 83345792, 84628032,
 85910272, 87192512, 88474752, 89756992, 91039232, 92321472, 93603712, 94885952, 96168192, 97450432, 98732672, 100014912, 101297152, 102579392, 103861632, 105143872, 106426112, 107708352, 108990592,
 110272832, 111555072, 112837312, 114119552, 115401792, 116684032, 117966272, 119248512, 120530752, 121812992, 123095232, 124377472, 125659712, 126941952, 128224192, 129506432, 130788672, 132070912,
 133353152
 
root@bhyve-1-10:~ # df -h
Filesystem                                                  Size    Used   Avail Capacity  Mounted on
/dev/vtbd0p2                                                 62G    5.3G     51G     9%    /
devfs                                                       1.0k    1.0k      0B   100%    /dev
tmpfs                                                       3.7G    4.0k    3.7G     0%    /tmp