qemuMonitorJSONGetMigrationCapabilities: Don't return early on CommandNotFound

qemuMonitorJSONGetMigrationCapabilities: Don't return early on CommandNotFound

The qemuMonitorJSONGetMigrationCapabilities() command executes
'query-migrate-capabilities' command and returns early if QEMU
doesn't know the command. Well, the command was introduced in
QEMU release 1.2 (specifically in commit v1.2.0-rc0~29^2~11) and
since the minimum required version is 2.11.0 we can be sure that
command will always exist.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>