x86/p2m: use large pages for MMIO mappings

x86/p2m: use large pages for MMIO mappings

When mapping large BARs (e.g. the frame buffer of a graphics card) the
overhead of establishing such mappings using only 4k pages has,
particularly after the XSA-125 fix, become unacceptable. Alter the
XEN_DOMCTL_memory_mapping semantics once again, so that there's no
longer a fixed amount of guest frames that represents the upper limit
of what a single invocation can map. Instead bound execution time by
limiting the number of iterations (regardless of page size).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>