Implement persistent UEFI settings using `BHYVE_UEFI_VARS`

[yaroslav-gwit]

Looks like in FreeBSD RELEASE v14 we have the ability to save the UEFI settings into a separate UEFI firmware file:

bhyve -c 2 -m 4g -w -H \\
  -s 0,hostbridge \\
  -s 31,lpc -p com1,stdio \\
  -l bootrom,/usr/local/share/uefi-firmware/BHYVE_UEFI_CODE.fd,BHYVE_UEFI_VARS.fd

I'll add a new config flag called persistent_uefi_vars (true or false) to activate and use it with Hoster, but it will not be default, because it's not available in FreeBSD RELEASE v13.

Also, keep in mind that BHYVE_UEFI_VARS.fd needs to be copied into the VM dataset's folder (one copy per VM) in order to be used (Hoster will do it automatically, of course), so at some point you might want to remove it in order to migrate to a newer file version automatically.