qemu: snapshot: Require support of 'transaction' command for external snapshots

qemu: snapshot: Require support of 'transaction' command for external snapshots

While qemu supports the 'transaction' command since v1.1.0
(52e7c241ac766406f05fa) and the 'blockdev-snapshot-sync' command since
v0.14.0-rc0 we need to keep the capability bits present since some qemu
downstreams (RHEL/CentOS 7 for example) chose to cripple qemu by
arbitrarily compiling out some stuff which was already present at that
time.

To simplify the crazy code just require both commands to be present at
the beginning of an external snapshot so that we can remove the case when
'transaction' would not be supported.

This also allows to drop any logic connected to the
VIR_DOMAIN_SNAPSHOT_CREATE_ATOMIC flag since snapshots are atomic with
the 'transaction' command.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>