qemuPrepareNVRAM: Us virFileRewrite() to write NVRAM
qemuPrepareNVRAM: Us virFileRewrite() to write NVRAM

After previous commits there is no need for qemuPrepareNVRAM() to
open code virFileRewrite(). Deduplicate the code by calling the
function.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

libvirt/libvirt,jfehlig/libvirt,zippy2/libvirt,jfehlig/libvirt,olafhering/libvirt,jfehlig/libvirt,jfehlig/libvirt,crobinso/libvirt,olafhering/libvirt,libvirt/libvirt,crobinso/libvirt,libvirt/libvirt,crobinso/libvirt,zippy2/libvirt,olafhering/libvirt,olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,crobinso/libvirt,zippy2/libvirt