vmx: Get the VMware boolean uefi.secureBoot.enabled

vmx: Get the VMware boolean uefi.secureBoot.enabled

Some VMware guests have a boolean uefi.secureBoot.enabled.  If found,
and it's set to "TRUE", and if it's a UEFI guest, then add this clause
into the domain XML:

  <os firmware='efi'>
    <firmware>
      <feature enabled='yes' name='enrolled-keys'/>
      <feature enabled='yes' name='secure-boot'/>
    </firmware>
  </os>

This approximates the meaning of this VMware flag.

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Fixes: https://issues.redhat.com/browse/RHEL-67836
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>