qemu: Prepare hostdev data which depends on the host state separately

qemu: Prepare hostdev data which depends on the host state separately

SCSI hostdev setup requires querying the host os for the actual path of
the configured hostdev. This was historically done in the command line
formatter. Our new approach is to split out this part into
'qemuProcessPrepareHost' which is designed to be skipped in tests.

Refactor the hostdev code to use this new semantics, and add appropriate
handlers filling in the data for tests and the qemuConnectDomainXMLToNative
users.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>