xen/common: event_channel: Don't ignore error in get_free_port()

xen/common: event_channel: Don't ignore error in get_free_port()

Currently, get_free_port() is assuming that the port has been allocated
when evtchn_allocate_port() is not return -EBUSY.

However, the function may return an error when:
    - We exhausted all the event channels. This can happen if the limit
    configured by the administrator for the guest ('max_event_channels'
    in xl cfg) is higher than the ABI used by the guest. For instance,
    if the guest is using 2L, the limit should not be higher than 4095.
    - We cannot allocate memory (e.g Xen has not more memory).

Users of get_free_port() (such as EVTCHNOP_alloc_unbound) will validly
assuming the port was valid and will next call evtchn_from_port(). This
will result to a crash as the memory backing the event channel structure
is not present.

Fixes: 368ae9a05fe ("xen/pvshim: forward evtchn ops between L0 Xen and L2 DomU")
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>