qxl: Don't drop client capability bits

qxl: Don't drop client capability bits

interface_set_client_capabilities() copies only the first few bits,
because it falls into a Classic C trap: you can declare a parameter
uint8_t caps[58], but the resulting parameter type is uint8_t *, not
uint8_t[58].  In particular, sizeof(caps) is sizeof(uint8_t *), not
the intended sizeof(uint8_t[58]).

Harmless, because the bits aren't used, yet.  Broken in commit
c10018d6.  Spotted by Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>