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

Update mshv_store_regs to use the register page when it is mmapped and
valid to set registers.
Otherwise use the ioctls to set the registers.

Signed-off-by: Doru Blânzeanu <dblanzeanu@linux.microsoft.com>
Reviewed-By: Magnus Kulke <magnuskulke@linux.microsoft.com>
Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
Link: https://lore.kernel.org/r/20260428135053.251200-7-dblanzeanu@linux.microsoft.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>