qemu_domain: add a PPC64 memLockLimit helper

qemu_domain: add a PPC64 memLockLimit helper

There is a lot of documentation in the comments about how PPC64 handles
passthrough VFIO devices to calculate the @memLockLimit. And more will
be added with the PPC64 NVLink2 support code.

Let's remove the PPC64 code from qemuDomainGetMemLockLimitBytes()
body and put it into a helper function. This will simplify the
flow of qemuDomainGetMemLockLimitBytes() that handles all the other
platforms and improves readability of the PPC64 specifics.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>