qemu: Introduce qemuBuildHostdevCommandLine

qemu: Introduce qemuBuildHostdevCommandLine

Add new function to manage adding the host device options to the
command line removing that task from the mainline qemuBuildCommandLine.

Also modify qemuBuildPCIHostdevDevStr, qemuBuildUSBHostdevDevStr,
and qemuBuildSCSIHostdevDevStr to use const virDomainDef instead
of virDomainDefPtr.

Make qemuBuildPCIHostdevPCIDevStr and qemuBuildUSBHostdevUSBDevStr
static to the qemu_command.c.

Signed-off-by: John Ferlan <jferlan@redhat.com>