qemu: command: move PSeries features validation to qemu_domain.c

qemu: command: move PSeries features validation to qemu_domain.c

Introduce a new function called qemuDomainDefValidatePSeriesFeature()
that will center all the PSeries validation done in qemu_command.c.
qemuDomainDefValidatePSeriesFeature() is then called during domain
define time, in qemuDomainDefValidateFeatures().

qemuxml2argvtest.c is also changed to include all the caps that now
are being validated in define time.

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>