qemu: don't 'remove' hostdev objects from domain if operation fails

qemu: don't 'remove' hostdev objects from domain if operation fails

There were certain paths through the hostdev detach code that could
lead to the lower level function failing (and not removing the object
from the domain's hostdevs list), but the higher level function
free'ing the hostdev object anyway. This would leave a stale
hostdevdef pointer in the list, which would surely cause a problem
eventually.

This patch relocates virDomainHostdevRemove from the lower level
functions qemuDomainDetachThisHostDevice and
qemuDomainDetachHostPciDevice, to their caller
qemuDomainDetachThisHostDevice, placing it just before the call to
virDomainHostdevDefFree. This makes it easy to verify that either both
operations are done, or neither.

NB: The "dangling pointer" part of this problem was introduced in
commit 13d5a6, so it is not present in libvirt versions prior to
0.9.9. Earlier versions would return failure in certain cases even
though the the device object was removed/deleted, but the removal and
deletion operations would always both happen or neither.