conf: Move hypervisor specific nhugepage checks

conf: Move hypervisor specific nhugepage checks

Commit 82327038 moved a couple of checks out of the XML parser
into the domain validation; however, those checks seem to be more
useful as hypervisor specific checks rather than the more general
domain conf checks (nothing in the docs indicate a specific error).

Fortunately only QEMU was processing the memoryBacking, thus
add the changes to qemuDomainDefValidateMemory and change the
code a bit to make usage of the similar deref to def->mem and
the mem->nhugepages filter.

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>