The internal Xen x86 emulator is fixed to handle shared/sharable pages corretly.

The internal Xen x86 emulator is fixed to handle shared/sharable pages corretly.
If pages cannot be unshared immediately (due to lack of free memory required to
create private copies) the VCPU under emulation is paused, and the emulator
returns X86EMUL_RETRY, which will get resolved after some memory is freed back
to Xen (possibly through host paging).

Signed-off-by: Grzegorz Milos <Grzegorz.Milos@citrix.com>