qemuAppendLoadparmMachineParm: add loadparm from hostdev

qemuAppendLoadparmMachineParm: add loadparm from hostdev

Commit 54fa1b44afc ("conf: Add loadparm boot option for a boot device")
added the ability to specify a loadparm parameter on a <boot/> tag, while
commit 29ba41c2d40 ("qemu: Add loadparm to qemu command line string")
added that value to the QEMU "-machine" command line parameters.

Unfortunately, the latter commit only looked at disks and network
devices for boot information, even though anything with
VIR_DOMAIN_DEF_FORMAT_ALLOW_BOOT could potentially have this tag.
In practice, a <hostdev> tag pointing to a passthrough (SCSI or DASD)
disk device can be used in this way, which means the loadparm is
accepted, but not given to QEMU.

Correct this, and add some XML/argv tests.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>