qemu-iotests: move "check" code out of common.rc

Some functions in common.rc are never used by the tests.  Move
them out of that file and into common, which is already included
only by "check".

Code that actually *is* common to "check" and tests can be placed in
common.config.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
