viridian: fix assertion failure

viridian: fix assertion failure

Whilst attempting to crash an apparently wedged Windows domain using
'xen-hvmcrash' I managed to trigger the following ASSERT:

(XEN) Assertion '!vp->ptr' failed at viridian.c:607

with stack:

(XEN)    [<ffff82d08032c55d>] viridian_map_guest_page+0x1b4/0x1b6
(XEN)    [<ffff82d08032b1db>] viridian_synic_load_vcpu_ctxt+0x39/0x3b
(XEN)    [<ffff82d08032b90d>] viridian.c#viridian_load_vcpu_ctxt+0x93/0xcc
(XEN)    [<ffff82d0803096d6>] hvm_load+0x10e/0x19e
(XEN)    [<ffff82d080274c6d>] arch_do_domctl+0xb74/0x25b4
(XEN)    [<ffff82d0802068ab>] do_domctl+0x16f7/0x19d8

This happened because viridian_map_guest_page() was not written to cope
with being called multiple times, but this is unfortunately exactly what
happens when xen-hvmcrash re-loads the domain context (having clobbered
the values of RIP).

This patch simply makes viridian_map_guest_page() return immediately if it
finds the page already mapped (i.e. vp->ptr != NULL).

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>