qemu: Warn verbosely if using old loader:nvram pairs

qemu: Warn verbosely if using old loader:nvram pairs

There are two ways for specifying loader:nvram pairs:

  1) --with-loader-nvram configure option
  2) nvram variable in qemu.conf

Since we have FW descriptors, using this old style is
discouraged, but not as strong as one would expect. Produce more
warnings:

  1) produce a warning if somebody tries the configure option
  2) produce a warning if somebody sets nvram variable and at
     least on FW descriptor was found

The reason for producing warning in case 1) is that package
maintainers, who set the configure option in the first place
should start moving towards FW descriptors and abandon the
configure option. After all, the warning is printed into config
output only in this case.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1763477

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>