target/i386/mshv: use the register page to get registers

Change the mshv_load_regs to use the register page when it is mmapped
and is valid.
Otherwise use the existing logic that uses ioctls to fetch registers.

When retrieving the special registers, there are some registers that are
not present in the register page: TR, LDTR, GDTR, IDTR, CR2, APIC_BASE.
For this ones we still need to use ioctls to correctly fetch.

Signed-off-by: Doru Blânzeanu <dblanzeanu@linux.microsoft.com>
Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
Link: https://lore.kernel.org/r/20260428135053.251200-6-dblanzeanu@linux.microsoft.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>