qemu: be sure we're using the updated value of backend during hotplug

qemu: be sure we're using the updated value of backend during hotplug

commit f094aaac changed qemuPrepareHostdevPCIDevices() such that it
may modify the "backend" (vfio vs. legacy kvm) setting in the
virHostdevDef. However, qemuDomainAttachHostPciDevice() (used by
hotplug) copies the backend setting into a local *before* calling
qemuPrepareHostdevPCIDevices(), and then later makes a decision based
on that pre-change value.

The result is that, if the backend had been set to "default" (i.e. not
specified in the config) and was later updated to "VFIO" by
qemuPrepareHostdevPCIDevices(), the qemu process' MacMemLock is not
increased (as is required for VFIO device assignment).

This patch delays making the local copy of backend until after its
potential modification.