qemu_domain: Move internals of qemuDomainPrepareHostdev() into a separate function

qemu_domain: Move internals of qemuDomainPrepareHostdev() into a separate function

So far, qemuDomainPrepareHostdev() is a NOP for anything but a
SCSI hostdev. This will change soon. Therefore, move the SCSI
hostdev preparation into a separate function
(qemuDomainPrepareHostdevSCSI()) and make
qemuDomainPrepareHostdev() call function corresponding to the
hostdev type (or nothing if the type doesn't need any
preparation).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>