qemu: Use qemuDomainRequiresMlock() when attaching PCI hostdev

qemu: Use qemuDomainRequiresMlock() when attaching PCI hostdev

The function is used everywhere else to check whether the locked
memory limit should be set / updated, and it should be used here
as well.

Moreover, qemuDomainGetMlockLimitBytes() expects the hostdev to
have already been added to the domain definition, but we only do
that at the end of qemuDomainAttachHostPCIDevice(). Work around
the issue by adding the hostdev before adjusting the locked memory
limit and removing it immediately afterwards.