qemu: snapshot: Ensure that NVRAM image exists when taking inactive internal snapshot

qemu: snapshot: Ensure that NVRAM image exists when taking inactive internal snapshot

Attempting to take an internal snapshot of a freshly defined VM with
qcow2 backed NVRAM results in failure as the NVRAM image doesn't get
populated until the VM is started for the first time.

Fix this by invoking qemuPrepareNVRAM() when qcow2 nvram is defined.

Resolves: https://issues.redhat.com/browse/RHEL-73315
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>