bhyve: fix vCPU single-stepping on VMX
This patch fixes virtual machine single stepping on VMX hosts.
Currently, when using bhyve's gdb stub, each attempt at single-stepping
a vCPU lands in a timer interrupt. The current single-stepping mechanism
uses the Monitor Trap Flag feature to cause VMEXIT after a single
instruction is executed. Unfortunately, the SDM states that MTF causes
VMEXITs for the next instruction that gets executed, which is often not
what the person using the debugger expects. 
This patch adds a new VM capability that masks interrupts on a vCPU by
blocking interrupt injection and modifies the gdb stub to use the newly
added capability while single-stepping a vCPU.
 Intel SDM 26.5.2 Vol. 3C
Reviewed by: corvink, jbh
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39949