qemu: simplify calling qemuDomainHostdevNetConfigRestore

qemu: simplify calling qemuDomainHostdevNetConfigRestore

This function was called in three places, and in each the call was
qualified by a slightly different conditional. In reality, this
function should only be called for a hostdev if all of the following
are true:

  1) mode='subsystem'
  2) type='pci'
  3) there is a parent device definition which is an <interface>
     (VIR_DOMAIN_DEVICE_NET)

We can simplify the callers and make them more consistent by checking
these conditions at the top ov qemuDomainHostdevNetConfigRestore and
returning 0 if one of them isn't satisfied.

The location of the call to qemuDomainHostdevNetConfigRestore() has
also been changed in the hot-plug case - it is moved into the caller
of its previous location (i.e. from qemuDomainRemovePCIHostDevice() to
qemuDomainRemoveHostDevice()). This was done to be more consistent
about which functions pay attention to whether or not this is one of
the special <interface> hostdevs or just a normal hostdev -
qemuDomainRemoveHostDevice() already contained a call to
networkReleaseActualDevice() and virDomainNetDefFree(), so it makes
sense for it to also handle the resetting of the device's MAC address
and vlan tag (which is what's done by
qemuDomainHostdevNetConfigRestore()).