qemu: Emit correct audit message for memory hot unplug

qemu: Emit correct audit message for memory hot unplug

https://bugzilla.redhat.com/show_bug.cgi?id=1226234#c3

If the qemu monitor fails to remove the memory from the guest for
any reason, the auditlog message will incorrectly use the current
actual memory (via virDomainDefGetMemoryActual) instead of the
value we were attempting to reduce to. The result is the 'new-mem'
and 'old-mem' values for the auditlog message would be identical.

This patch creates a local 'newmem' which accounts for the current
memory size minus the memory which is being removed. NB, for the
success case this results in the same value that would be returned
by virDomainDefGetMemoryActual without the need to do the math. This
follows the existing code which would subtract the size for cur_balloon.

Signed-off-by: Luyao Huang <lhuang@redhat.com>