qemu: Replace @dom argument with @driver in qemuDomainUpdateDeviceLive()

qemu: Replace @dom argument with @driver in qemuDomainUpdateDeviceLive()

The qemuDomainUpdateDeviceLive() accepts virDomainPtr as one of
its arguments, but use it only to get QEMU driver out of it.
Well, the only caller already does that and thus can pass it
instead of virDomainPtr.

This also makes it look like the rest of device hot(un-)plug
functions: qemuDomainAttachDeviceLive() and
qemuDomainUpdateDeviceLive().

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Kristina Hanicova <khanicov@redhat.com>