qemu: Use qemuDomainOpenFile() in qemuPrepareNVRAM()

qemu: Use qemuDomainOpenFile() in qemuPrepareNVRAM()

Previously, nvram file was created with user/group owner as
'root', rather than specifications defined in libvirtd.conf. The
solution is to call qemuDomainOpenFile(), which creates file with
defined permissions and qemuSecurityDomainSetPathLabel() to set
security label for created nvram file.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1783255

Signed-off-by: Kristina Hanicova <khanicov@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>