Allow guest device MMIO access from bootmem memory segments.

Recent versions of UEFI have moved local APIC timer initialization into
the early SEC phase which runs out of ROM, prior to self-relocating
into RAM. This results in a hypervisor exit.

Currently bhyve prevents instruction emulation from segments that aren't
marked as "sysmem" aka guest RAM, with the vm_gpa_hold() routine failing.
However, there is no reason for this restriction: the hypervisor already
controls whether EPT mappings are marked as executable.

Fix by dropping the redundant check of sysmem.

MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D25955