Move QEMU caps validation of qemuBuildHostdevCommandLine() to
qemuDomainDeviceDefValidateHostdev() and qemuDomainMdevDefValidate(),
allowing them to be validated at domain define time.

Tests were adapted to consider the new caps being needed in
this earlier stage.
Reviewed-by: NCole Robinson <crobinso@redhat.com>
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>