qemu_passt: Report passt's error on failed start

qemu_passt: Report passt's error on failed start

When starting passt, it may write something onto its stderr
(convincing it to print even more is addressed later). Pass this
string we read to user.

Since we're not daemonizing passt anymore (see previous commit),
we can let virCommand module do all the heavy lifting and switch
to virCommandSetErrorBuffer() instead of reading error from an
FD.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>