qemu: Move parts of NIC_RX_FILTER_CHANGED event handling into a function

qemu: Move parts of NIC_RX_FILTER_CHANGED event handling into a function

Parts of the code that responds to the NIC_RX_FILTER_CHANGED
event are going to be re-used. Separate them into a function
(qemuDomainSyncRxFilter()) and move the code into qemu_domain.c
so that it can be re-used from other places of the driver.

There's one slight change though: instead of passing device alias
from the just received event to qemuMonitorQueryRxFilter(), I've
switched to using the alias stored in our domain definition. But
these two are guaranteed to be equal. virDomainDefFindDevice()
made sure about that, if nothing else.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>