x86/hvm: simplify 'mmio_direct' check in epte_get_entry_emt()

x86/hvm: simplify 'mmio_direct' check in epte_get_entry_emt()

Re-factor the code to take advantage of the fact that the APIC access page is
a 'special' page. The VMX code is left alone and hence the APIC access page is
still inserted into the P2M with type p2m_mmio_direct. This is left alone as it
is not obvious there is another suitable type to use, and the necessary
re-ordering in epte_get_entry_emt() is straightforward.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>