iotests: Skip 211 on insufficient memory

iotests: Skip 211 on insufficient memory

VDI keeps the whole bitmap in memory, and the maximum size (which is
tested here) is 2 GB.  This may not be available on all machines, and it
rarely is available when running a 32 bit build.

Fix this by making VM.run_job() return the error string if an error
occurred, and checking whether that contains "Could not allocate bmap"
in 211.  If so, the test is skipped.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190218180646.30282-1-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>