KVM: interrupt kvm_gmem_populate() on signals

kvm_gmem_populate() is a potentially lengthy operation that can involve
multiple calls to the firmware.  Interrupt it if a signal arrives.

Fixes: 1f6c06b1 ("KVM: guest_memfd: Add interface for populating gmem pages with user data")
Cc: Isaku Yamahata <isaku.yamahata@intel.com>
Cc: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>